feat(devkitd): migrate agent group management to daemon.db #413

Part of the devkitd agent migration (#406). Target: group management in daemon.db.

Scope

agent group * (create/update/add/remove-agent/remove/list/detail):

• groups table + membership in daemon.db
• group.* RPCs for all 7 subcommands
• agent send --group → group.lookup → per-member agent.lookup + client-side sends
• Migrate existing group data (one-time import, same pattern as legacy agents bridge)

Acceptance

• All agent group subcommands daemon-routed
• send --group resolves membership via RPC